1. Field of the Invention
The invention relates in general to a scanner, and more particularly to a duplex scanner having two scanning modules.
2. Description of the Related Art
Along with the rapid advance in technology, the integration and application of multi-media have become more and more popular. Multi-media refer to data contents of various data types such as texts, images, sounds etc. In order to present data through multi-media, various devices for capturing images or sounds have been developed. For instance, scanner is one of most commonly used tools for capturing images.
Referring to FIG. 1A, a block diagram of a conventional sheet-fed type simplex scanner with a stepper motor is shown. In FIG. 1A, scanner 10 comprises a charge-coupled device (CCD) 11, a controller 12, a stepper motor 13 and a roller set 14. When the scanner 10 starts to scan a to-be-scanned document (not shown in FIG. 1A), firstly, the controller 12 controls the stepper motor 13 to run so as to drive the roller set 14 to move the to-be-scanned document. Next, the controller 12 controls the CCD 11 to capture an image of the to-be-scanned document. When the scanner 10 incorporates a document turning mechanism, the scanner 10 can scan both a front side and a back side of the to-be-scanned document to achieve duplex scan.
Referring to FIG. 1B, a block diagram of a conventional flat-bed type simplex scanner with a stepper motor is shown. The scanner 15 comprises a CCD 16, a controller 17, a stepper motor 18 and a roller set 19. When the scanner 15 starts to scan a to-be-scanned document (not shown in FIG. 1B), the controller 17 controls the stepper motor 18 to run so as to drive the roller set 19 to move the CCD 16, so that CCD 16 can capture the image of the to-be-scanned document. When the scanner 15 incorporates a document turning mechanism, the scanner 15 can scan both a front side and a back side of the to-be-scanned document to achieve duplex scan. However, the document turning mechanism of either type of the simplex scanners would only increase scanning time.
The stepper motor adopts an open-loop control method, and will easily lose step-synchronism when exceeding allowed pull-in torque due to a high rotation rate. If the stepper motor loses the step-synchronism for too long, a motor stall will occur and becomes an unsolvable problem. Besides, if the stepper motor adopts micro-stepping driving method, the positional accuracy is normally hard to achieve the required smooth level. Besides, due to limitation imposed by the internal structure of the stepper motor, a stepper motor normally is not functional at a rotational speed over 5000 rpm, and has poor torque characteristics at any high rotational speed.
To maintain a maximum holding torque, an open-loop controlled stepper motor is normally operated under fully powered state, resulting in a high operating temperature. The stepper motor has significant noises and mechanic vibration problems when operating in full step state, especially at a low rotational speed. Moreover, the resonant band of the stepper motor is too many and too wide, such that when other mechanisms are incorporated, resonance and noises are easily induced. The open-loop controlled stepper motor will be inadequate to accommodate scanner's future requirements, such as high rotational speed, high load, high load variation, little vibration, low noises and high scanning speed.